A directional coupler is a four-port device that enables measurement of power of an input signal. The four ports may be labeled input port, output port, forward coupled port and the reverse coupled port. The input and output ports connect to a device under test (DUT), for example, and the forward and reverse coupled ports are used for monitoring power. The signal at the forward coupled port is proportional to the signal traveling in a forward direction, from the input port to the output port (e.g., input signal). The signal at the reverse coupled port is proportional to the signal traveling in a reverse direction, from the output port to the input port (e.g., reflected signal).
One common application of a directional coupler is monitoring power between a radio transmitter and an antenna in a radio system, for example, where the transmitter and the antenna are connected to the input port and the output port of the directional coupler, respectively. Power flows from the transmitter to the antenna (forward power), and thus from the input port to the output port. When the antenna is imperfect, some of the power reflects off the antenna (reverse power) and flows back toward the input port, returning to the radio transmitter. This is undesirable for at least two reasons. First, the reverse power reduces the amount of power radiated from the antenna, thus reducing range and sensitivity of the radio system. Second, an excessive amount of the reverse power may damage the transmitter. Therefore, antenna designs attempt to minimize reverse power.
FIG. 1 is a simplified block diagram of a directional coupler. Referring to FIG. 1, directional coupler 110 includes transmission line 111 having a first port 101 (input port) for receiving an input signal, e.g., from a radio transmitter, and a second port 102 (output port) for outputting the input signal, e.g. to an antenna. The directional coupler 110 also includes coupled line 112 having a third port 103 (forward coupled port) for presenting sampled power of the input signal flowing from the first port 101 to the second port 102, and a fourth port 104 (reverse coupled port) for presenting sampled power of a reflected signal (reflected from a load connected to the second port 102) flowing from the second port 102 to the first port 101. The fourth port 104 may also be referred to as an isolated port with regard to the input signal, and the third port 103 may be referred to as an isolated port with regard to the reflected signal. As mentioned above, the directional coupler 110 has the property that the power of the coupled signal measured at the third port 103 is proportional to the forward power, flowing from the first port 101 to the second port 102. Similarly, the power of the coupled signal measured at the fourth 104 is proportional to the reverse power, flowing from second port 102 to the first port 101. Thus, by measuring the power of the coupled signals at the third and fourth ports 103 and 104, the forward power and reverse power flowing between the transmitter and the antenna may be determined, respectively.
Power traveling between any two ports of the directional coupler 110 may be indicated using S-parameters, as is know in the art, where the first port 101 is port “1,” the second port 102 is port “2,” the third port 103 is port “3” and the fourth port 104 is port “4.” Thus, the ratio between the power at the third port 103 and the forward power of the input signal, which may be referred to as the “coupling factor,” may be indicated by S31 in S-parameter terminology. In addition, S31 is a measure of the sensitivity at the third port 103 to the forward power, and S32 is a measure of the sensitivity at the third port 103 to the reverse power. The ratio between S-parameters S32 and S31 may be referred to as “directivity.” Accordingly, the S-parameters of the directional coupler 110 with respect to coupling factor and directivity may be indicated as follows:S31=C S32=C*D S42=C S41=C*D S12=S21≈1
In an ideal directional coupler, the third port 103 outputs only a coupled signal that is proportional to the forward power, and is not affected at all by the reverse power. Likewise, the fourth port 104 ideally outputs only a coupled signal that is proportional to the reverse power, and is not affected at all by the forward power. Of course, no actual directional coupler is ideal, so in practice the third port 103 actually outputs a coupled signal that includes both a desired coupled signal that is proportional to the forward power and an extraneous coupled signal that is proportional to the reverse power, and the fourth port 104 also outputs a coupled signal that includes both a desired coupled signal that is proportional to the reverse power and an extraneous coupled signal that is proportional to the forward power. The extraneous coupled signals negatively affect directivity.
Some conventional directional couplers attempt to limit extraneous coupled signals and improve directivity through manual tuning during production, which is time consuming and inflexible. For example, some conventional directional couplers include tuning blocks that are shifted to achieve desired directivity, and then glued in place. This process is time consuming in that the coupler lid must be removed repeatedly to adjust and readjust the tuning blocks, but replaced each time to measure directivity. Further, once the tuning blocks are set, the directional coupler is effectively limited to the frequency at which the tuning occurred. Similarly, some conventional directional couplers include metal tuning slugs that are threaded through the body of the directional coupler. Since the tuning slugs can be accessed from the outside, the coupler lid does not need to be removed for tuning. However, the manual alignment is still time consuming and cannot be easily readjusted for handling input signals having different frequencies.
Accordingly, there is a need to improve directivity of directional couplers, particularly by reducing or eliminating the effects of reverse power on the output of the third port 103, as well as by reducing or eliminating the effects of forward power on the output of the forth port 104. Generally, improving the directivity of a coupler enables more accurate measurements of the forward power and/or reverse power.